Luminescence cooperative

The processes involved are stepwise energy transfer, cooperative sensitization of luminescence and cooperative luminescence. As an example of stepwise energy transfer a system containing Er and Yb may be considered. The latter ion absorbs at 970 nm (10,300 cm ) and in phonon assisted an Er ion... 

If an ion possesses two excited states with approximately same energy separation, energy transfer can occm between two ions in the first excited state (Figme 4c). One ion returns uomadiatively to the ground state while the second is promoted to the second excited state, then decays with emission of a photon whose energy is about twice that of incident photons. There are other upconversion processes two-step absorption, cooperative sensitization, cooperative luminescence. Upconversion by energy transfer is the most efficient process. 

Cooperative luminescence is the microscopic reverse of simultaneous pair excitation, and in Yb + involves the emission of a green photon following NIR Fy/2 Fj/2 excitation by the simultaneous relaxation of two excited ions to the ground state. In general, cooperative upconversion systems can be treated by the same rate equations as those given in Eq. (10), where now E = G, k2b = Ik -, and the cooperative luminescence rate constant is k oop = 2a Under low-power conditions, where Gsteady-state cooperative luminescence rates in the limit of a purely radiative system are described by Eqs. (29) and (30) ... 

Fig. 14 a, b. Power and temperature dependence of a the visible cooperative luminescence b the NIR downconversion luminescence in 10% Yb + Cs3Lu2Br9 excited at 10,591 cm plotted on linear axes. These data show distinct hystereses, the properties of which are temperature dependent. Inset 10 K laser transmission, which also shows hysteresis behavior. Adapted from [62]... 

As described in Sect. 9, photon avalanches are only observed in cases where ai > Oq. In treating this case as an avalanche, the following question arises How does an ESA cross section become substantially greater than the GSA cross section in a cooperative luminescence system in which by definition the GSA and ESA steps are the same single-ion transition The answer to this question is... 

Schaudel B, Goldner P, Prassas M, Auzel F (2000) Cooperative luminescence as a probe of clustering in Yb doped glasses. J Alloys Compd 300-301 443 49... 

However, reality is not that simple and different up-conversion processes may exist simultaneously, or their effects can be tentatively reinforced reciprocally. For instance, two-photon absorption and cooperative absorption have been theoretically investigated (Rios Leite and de Araujo 1980). Also SHG and cooperative luminescence have been considered simultaneously, in order to increase SHG by the partial resonance of cooperative luminescence (Bonneville and Auzel 1976, Ovsyankin and Fedorov 1981). 

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